Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD) are two commonly used techniques for allowing two way communication.
The TDD system uses a single RF channel for allowing communication and hence can be considered similar to a simplex channel in that transmission takes place in one direction at a time, and then switches direction (e.g. from transmission to reception). However in TDD systems the switching from transmission. (Tx) to reception (Rx) is extremely fast and is transparent to the user, giving the impression of a duplex link.
The FDD system on the other hand uses two RF channels, usually well seperated in frequency, one for each direction of transmission and thus provides for dual simplex operation.
Both systems effectively enable duplex, two way voice communications. There are, however, differences in the implementation of such systems that give advantages and disadvantages to each system depending on the deployment of the system.
For TDD a period has to be allowed between transmission and reception to allow for the time it takes for the radio wave to travel between the two sites. This reduces the amount of information that can be transmitted over the link and thus limits the maximum range of the system. It also means that for cellular radio, multiple base stations within communications range of a mobile have to be synchronized in time.
For an FDD system, different frequencies are used for uplink and downlink transmitters, so the system can operate in a less synchronised manner.
To maximize the performance of a system a technique known as diversity reception is often used. In a cellular radio system it is often not practical to provide diversity at the handset. However, a base station can use uplink (mobile to base station) diversity for reception. In a TDD system, unlike an FDD system, downlink (Base to Mobile) diversity can be provided by using the uplink diversity selection technique in reverse. This is because with TDD, providing there is a sufficiently short timescale between transmit and receive periods, the path between the base and mobile will be relatively unchanged. (This allowable time period is primarily determined by the relative velocities of the two terminals and any multipath reflection points affecting the link).
For FDD systems the uplink and downlink are at differing frequencies and thus, the uplink diversity technique cannot be used in reverse as the uplink and downlink are affected differently by multipath reflection. Thus only uplink diversity is available to FDD systems.
For these reasons, TDD systems are generally optimum for environments where short range communications is required with particularly difficult propagation paths i.e. indoor systems or urban micro-cellular systems. Whereas FDD systems are more appropriate for long range outdoor systems i.e. Macrocellular systems where maximum cell ranges are required.
At present the needs of cellular radio are in general met by using long range FDD systems outdoor in the licenced band and using short range low power TDD system in the unlicenced band. These two networks normally require separate handsets and base stations.
There is, therefor, a need to provide for a single wireless network that can be optimized for both long range (Macrocells) together with indoor coverage and urban microcells.
There are also issues of compatibility between networks using the two duplexing methods unless care is taken in the network design. For example, usually there are two frequency bands allocated to PCS systems, called an upper band and a lower band. There are also multiple operators in each area which share these bands. For example, assuming two operators a typical allocation would give the first operator the first half of the lower band and also the first half of the upper band, while the second operator in the area gets the upper half of the lower band and the upper half of the upper band. While there is typically a relatively large separation between the upper band and the lower band there is potential for interference between the two operators within each band. If both operators use FDD this does not generally present a problem as each operator only uses their channels in the upper band for transmission from the base station whereas each operator uses their channels in the lower range for reception. However, if one operator uses FDD and another operator uses TDD then mutual interference is possible for transmission by closely located base stations. The increasing pressure on operators to share sites (to reduce Cost, Environmental Impact, Site Availability, Zoning requirements etc.) gives an advantage to a system that can adapt its duplexing means to suit the local radio environment. This can offer more flexibility for site sharing arrangements without the need for specific site engineering whilst retaining the advantages of both systems.
There is therefore a need to provide a system which is flexible enough to use either TDD or FDD depending on which is more favourable for the particular application.